A clock's Minute hand rotates once for each hour and the Hour hand rotates once for every 12 hours, so a ratio of 12:1 reduction is required from the gear train that connects the two hands. To do this I normally use two pairs of gears with 3:1 and 4:1 ratios giving the total reduction of 12:1 required.
I have used this design on nearly all of my clocks in the past. but there has always been another design that I could have incorporated. This is a concept devised by a Clockmaker called Aaron Dodd Crane around 1830 and uses a variation on the Hypocycloidal Speed Reducer.
The hypocycloidal Speed Reducer shown above is used to produce a very high torque output in a compact space. In this form, it has a couple of disadvantages. The first is that it requires extreme accuracy so as to avoid high friction and scraping, and as if this was not enough, it also has an output that runs in reverse of the input. This, of course, is of no use to the clock builder, who needs both Minute and Hour hands to run in the same direction.
The Daisy Wheel concept gets around both of these problems and is called the Daisy Wheel motion.
It has an inner ring of 11 teeth and an outer ring of 6 teeth, the teeth being none standard, as shown in the video here. The inner ring is constrained by the brass rod sliding in a rocking sleeve and adopts a rocking action to move the outer ring by rotating about an eccentric pivot. The motion is complex and almost mystical in the way that it works. If you watch the video below and concentrate on the Left-Hand side, you will see that each of the 4 teeth on the outer ring is given a little nudge by the top of the nearest Daisy petal, and this is happening at each of the teeth on the Outer Ring in turn causing almost continuous motion of the Outer Ring. You may also note that even though there are only 4 teeth on the Outer Ring and not 12, you still get a 12:1 reduction required for the Hour Hand. The silver rod hanging down from the Daisy engages in a two-fingered slot below so that the Daisy itself is restrained from rotating and simply rocks in place to provide the nudge needed to move the outer ring.
I have adopted this to use in a new clock project that incorporates the Daisy in this more literal sense and opens up the workings to be more visible on the front of the clock.
The main workings are mounted directly onto the front of the clock frame on the Minute shaft and are held in place by the Minute Hand when it is fitted to its shaft. I was a little unsure about mounting the Hour hand around the outside of the Outer Ring and it could have been mounted directly to the eccentric but then its ribs would have obscured part of the Daisy mechanism. However, seeing this arrangement with the Dial-in place convinced me that it would look and work OK.
The diagram below shows all the parts exploded so you can get a better view of each of the ports and how they relate to each other.
I shall be incorporating this design into my next clock coming in the spring, but if you would like to try using it in one of your own designs you can try it out with the files here
If you are interested in more information on Aaron Dodd Crane, extracts from the book 'Aaron Dodd Crane.An American original ' are shown below.
